The polymerization of styrene is a very important industrial process that supplies materials used to create a wide variety of polystyrene-containing articles. This expansive use of polystyrene results from the ability to control the polymerization process. Thus, variations in the polymerization process conditions are of utmost importance since they in turn allow control over the physical properties of the resulting polymer. The resulting physical properties determine the suitability of polystyrene for a particular use. For a given product, several physical characteristics must be balanced to achieve a suitable polystyrene material. Among the properties that must be controlled and balanced are averaged molecular weight (Mw) of the polymer, molecular weight distribution (MWD), melt flow index (MFI), and the storage modulus (G′).
U.S. Pat. No. 5,540,813 by Sosa, et. al., which is fully incorporated herein by reference, discloses a process for preparing monovinyl polymers, such as polystyrene, which utilizes a combination of sequentially ordered multiple reactors, heat exchangers and devolatilizers to strictly control polymer properties such as the molecular weight distribution and melt flow index.
The relationship between the molecular weight and the storage modulus is of particular importance in polymer foam applications. Such foam applications require high molecular weight polymers having a high storage modulus. It is thought that the storage modulus is related to the degree of branching along the polymer chain. As the degree of branching increases, the likelihood that a branch entangles with other polymer chains increases. A polymer product having a higher degree of branching or cross-linking tends to have a higher storage modulus, and therefore better foam stability characteristics.
Methods for preparing branched polymers are well-known in the art. For example, the preparation of branched polystyrene by free radical polymerization has been reported. Both methods increase the branching in the devolatilization step and produce a polymer with an undesirably low molecular weight.
Rather than employing free radical polymerization, some have used multi-functional mercaptans to form branched polymers. While materials having an acceptable molecular weight can be prepared by this method, these products are often unacceptable for foam applications with typical blowing agents due to their undesirable flow properties.
The properties of randomly branched polystyrene prepared in the presence of divinylbenzene have been reported by Rubens (L. C. Rubens, Journal of Cellular Physics, pp 311-320, 1965). However, polymers having a useful combination of molecular weight and cross-linking are not attainable. At low concentrations of divinylbenzene, low molecular weight polymers having little branching result. However, higher concentrations of the cross-linking agent result in excessive cross-linking and concomitant gel formation that is highly undesirable in industrial polystyrene processes. Similar results and problems were reported by Ferri and Lomellini (J. Rheol. 43(6), 1999).
A wide variety of peroxy compounds is known from the literature as initiators for the production of styrenic polymers. Commercially available initiators for polymer production may be classified in different chemical groups, which include diacylperoxides, peroxydicarbonates, dialkylperoxides, peroxyesters, peroxyketals, and hydroperoxides. Peroxides and hydroperoxides undergo at least four reactions in the presence of monomers or hydrocarbons with double bonds. These reactions are: 1) chain transfer, 2) addition to monomer, 3) hydrogen abstraction, and 4) re-combination, often called a cage effect.
Hydroperoxides have been shown to undergo induced decomposition reactions, in which a polymer radical (˜˜P*) will react with the initiator as shown below. This reaction is basically a chain transfer reaction and the reaction should be amenable to the well-known chain transfer equations. Radicals obtained from peroxide initiators (RCOO*) can also abstract a hydrogen from the hydroperoxide.RCOO* or ˜˜P*+RCOOH→˜˜PH+ROO*Baysal and Tobolsky (Journal of Polymer Science, Vol. 8, p. 529 et seq., (1952), fully incorporated by reference herein) investigated the chain transfer of polystyryl radicals to t-butyl hydroperoxide (t-BHP), cumyl hydroperoxide (CHP), benzoyl peroxide (Bz2O2), and azobisisobutyronitrile (AIBN). AIBN and benzoyl peroxide give the classical linear correlations between rate and 1/DP (Degree of Polymerization) indicating no chain transfer to initiators. The hydroperoxides, however, show significant levels of chain transfer.
A. I. Lowell and J. R. Price (Journal of Polymer Science, Vol. 43, p. 1, et seq. (1960), fully incorporated by reference herein) also showed that polystyryl radicals undergo considerable chain transfer with bis(2,4-dichloro) benzoyl peroxide as compared to dilauroyl peroxide.
Commercial polystyrene made by the conventional free-radical process yields linear structures and structures with low levels of branching. As noted, methods to prepare branched polystyrenes, however, are not easily optimized and few commercial non-linear polystyrenes are known. Studies of branched polymers show that these polymers possess unique molecular weight-viscosity relationships due to the potential for increased molecular entanglements. Depending upon the number and length of the branches, non-linear structures can give melt strengths equivalent to that of linear polymers at slightly higher melt flows.
U.S. Pat. No. 6,353,066 to Sosa describes a method of producing a copolymer by placing a vinylbenzene (e.g. styrene) in a reactor, placing a cross-linking agent (e.g. divinylbenzene) in the reactor, and placing a chain transfer agent (e.g. mercaptan) in the reactor and forming a polyvinyl benzene in the presence of the cross-linking agent and chain transfer agent. The melt strength of a polymer may also be improved by lightly cross linking a polymer.
It would be desirable if methods could be devised or discovered to provide vinyl polymers with increased branching, such as branched polystyrene with improved properties. It would also be helpful if a method could be devised that would help optimize the physical properties of vinyl polymers having increased branching. Such polymers may have higher melt strength than linear chains, and may improve processability and mechanical properties of the final product (e.g. lower density in foam applications).
While the preparation of branched polymers such as polystyrene by free radical polymerization has been reported, a need exists for a reliable method of measuring and quantifying branching in such polymers. It is also desirable to quantify branching in polymers to more clearly understand the effect of various additives used during polymerization reactions. In the art it is also desirable to provide vinyl polymers with increased branching, a satisfactory molecular weight, and a higher melt strength than linear chains for improved processability and mechanical properties in products (e.g. increased strength and/or lower density in foam). It is also desirable to determine how to best commercially produce polymers with optimal characteristics. Further, it is desirable to define the parameters of density and melt flow index for the optimal characteristics of foamed polymers.
A need also exists to explore the relationship between the molecular weights percentages of the linear fractions to the non-linear fractions in such polymers.